This invention relates to resins containing isocyanurate linkages and imide linkages and to methods for preparing them.
Resins containing imide or isocyanurate linkages are known to be useful in preparation of foamed resins for insulation applications as well as in the preparation of lightweight flame resistant structural foams for use in aircraft, packaging and the like. Such resins can be prepared by the reaction of polyisocyanates with dicarboxylic acids or anhydrides, e.g., see U.S. Pat. Nos. 3,314,923; 3,479,305; 3,562,189; 3,592,789; 3,644,234; 3,708,458 and 3,772,216. These linear polyimides are difficult to fabricate into useful items.
U.S. Pat. No. 3,933,700 describes the preparation of low molecular weight oligomers having viscosities as low as 400 cps at 75.degree. F. and resulting from the reaction between excess polyisocyanate and a dianhydride at reaction temperatures of 200.degree. C. A thermoset resin was obtained via the formation of urethane linkages by reacting the oligomer with a multi-functional, hydroxyl compound.
It is also known to prepare bisimide polymers from N,N'-bismaleimide as described in U.S. Pat. Nos. 3,380,964; 3,406,148 and 3,533,996. While it is easier to fabricate articles from the bisimide resins than from linear polyimides, the bisimide resins still do not have the processability required for many applications due to their potential to cross-link at temperatures above their melting points.
Recently as described in U.S. Pat. Nos. 4,108,810 and 4,110,274, cross-linked imide resins have been prepared by reacting an ethylenically unsaturated dicarboxylic acid and/or anhydride with a polyisocyanate. While such resins exhibit generally similar thermal resistance, they are also somewhat difficult to fabricate or otherwise process due to their increased solution or melt viscosities.
In view of the aforementioned problems with noncross-linked as well as cross-linked imide resins, it would be desirable to provide an imide-containing resin having thermal resistance equal to or better than that of the cross-linked resins and processability equal to or better than that of the noncross-linked imide resins described in U.S. Pat. Nos. 4,108,810 and 4,110,274. It also would be desirable to provide a resin that cures readily to a highly cross-linked state having excellent processability as well as thermal resistance.